This invention relates to the coating of major surfaces of elongated strip articles, such as sheet metal strip and the like. More particularly, it relates to apparatus and methods for continuously applying, to a strip major surface, a layer of paint or other liquid coating material.
By way of specific illustration, detailed reference will be made herein to the coating of sheet metal in greatly elongated strip form, as used for making siding panels for cladding exterior building walls, it being understood, however, that the invention in its broader aspects embraces the coating of other types of articles and surfaces as well.
In the production of siding panels from metal strip, at least one major surface of the strip is first given a protective and decorative coating of paint, and the strip is thereafter formed and cut into individual panels. As described, for example, in U.S. Pat. Nos. 4,356,217 and 4,411,218, the disclosures of which are incorporated herein by this reference, it is known to perform strip coating on a continuous line using a rigid coating head that defines an elongated open-sided slot to which liquid coating material (e.g. paint) is continuously supplied under pressure. The strip is continuously advanced longitudinally past the open side of the slot in a direction transverse to the long dimension of the slot while a major surface of the advancing article is maintained in facing proximate relation to the slot such that liquid coating material from the slot is deposited in a layer on the advancing strip surface. The thickness of this layer is dependent on the spacing between the strip surface and the coating head downstream of the slot; as shown in the aforementioned patents, the head is provided with an extended surface immediately adjacent the open side of the slot, and this surface defines (with the strip surface being coated) a metering orifice that determines the thickness of paint coating carried on the strip away from the slot. Typically, the strip advancing past the slot is backed up by an axially fixed roll.
The aforementioned patents particularly describe coating apparatus and procedures wherein the slot has the form of a trench and the applied paint or like coating has a striated or variegated appearance effected by concurrent supply of at least two colors of paint to the slot. The described apparatus and procedures, however, may also be employed to apply a single-color layer of paint.
In coating operations of this general type, desired uniformity of coating thickness requires maintenance of a metering orifice of invariant aperture; i.e., the spacing between the coating head and the coated strip surface, downstream of the slot, must remain constant despite variations in thickness of the strip being coated. Heretofore, it has been customary to control the thickness of coating application by mechanically fixing the position of the coating head surface relative to the strip surface being coated. In order to accommodate variations in strip thickness, in these known systems, the entire coating assembly has been mounted on small wheels that ride on the strip ahead of the locality of coating application (i.e., ahead of the slot). The use of such wheels has given rise to problems, especially in production of coatings of uniform single color. It is found that the wheels make visible marks that show through a single-color coating applied over a primer coating, although these marks are masked when the applied coating has a variegated or striated pattern; since use of a primer coating is generally desirable, and since the presence of such marks detracts from the appearance and acceptability of the coated product, the use of the wheels to maintain uniform spacing is disadvantageous for monochromatic coating applications. Also, in an experimental run on a strip-coating line employing the described wheels, bars of varying thickness across the width of the sheet have been observed; these bars are believed to have been caused by the wheels, either through vibration or as a result of the fact that the wheels must be spaced some distance ahead of the slot. Again, the creation of such bars detracts from desired coating uniformity.